Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus is arranged to project a pattern from a patterning device onto a substrate is disclosed. The lithographic apparatus includes an illumination system and an outlet connected to a pumping system to pump away gas from between an inner wall and outer wall of the illumination system or, if a radiation source is present, between the inner wall of the illumination system and an inner wall of the radiation source.

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/898,933, filed Sep. 17, 2007, now allowed, which isincorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). A patterning device, which is alternativelyreferred to as a mask or a reticle, may be used to generate a circuitpattern to be formed on an individual layer of the IC. This pattern canbe transferred onto a target portion (e.g. comprising part of, one, orseveral dies) on a substrate (e.g. a silicon wafer). Transfer of thepattern is typically via imaging onto a layer of radiation-sensitivematerial (resist) provided on the substrate. In general, a singlesubstrate will contain a network of adjacent target portions that aresuccessively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{{NA}_{PS}}}} & (1)\end{matrix}$

where λ is the wavelength of the radiation used, NA_(PS) is thenumerical aperture of the projection system used to print the pattern,k₁ is a process dependent adjustment factor, also called the Rayleighconstant, and CD is the feature size (or critical dimension) of theprinted feature. It follows from equation (1) that reduction of theminimum printable size of features can be obtained in three ways: byshortening the exposure wavelength λ, by increasing the numericalaperture NA_(PS) or by decreasing the value of k₁.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. An EUV radiation source isconfigured to output a radiation wavelength of about 13 nm. Thus, an EUVradiation source may constitute a significant step toward achievingsmall features printing. Such radiation is termed extreme ultraviolet orsoft x-ray, and possible sources include, for example, a laser-producedplasma source (LPP source), a discharge plasma source (DPP source), orsynchrotron radiation from an electron storage ring.

SUMMARY

Some types of EUV radiation sources, for instance a LPP source, producecontamination along with the EUV radiation. Some LPP sources have beenknown to produce HBr, in molecular and/or acidic form. Since thepressure in an EUV radiation source is generally high relative to thepressure in the lithographic apparatus or another part of thelithographic apparatus if the radiation source is part of thelithographic apparatus, the contamination will tend to enter andcontaminate an illumination system used with or part of the lithographicapparatus.

It is desirable, for example, to eliminate or reduce source-producedcontamination of the illumination system or other structures.

According to an aspect, there is provided a lithographic apparatusarranged to project a pattern from a patterning device onto a substrate,the lithographic apparatus comprising:

an illumination system configured to condition radiation from aradiation source, the illumination system having an inner wall facingaway from the radiation source and an outer wall facing toward theradiation source, the inner and outer walls having an aperture to allowradiation from the radiation source to pass into the illuminationsystem;

a pumping system configured to pump away gas;

an outlet connected to the pumping system, the outlet located at or nearthe aperture and located between the inner wall and the outer wall ofthe illumination system or, if the lithographic apparatus comprises aradiation source, between the inner wall of the illumination system andan inner wall of the radiation source facing toward the radiation sourceand adjacent the inner wall of the illumination system; and

a support structure constructed to hold the patterning device, thepatterning device being capable of imparting the radiation with apattern in its cross-section to form a patterned radiation beam.

According to an aspect, there is provided a radiation source arranged toemit radiation, the source comprising:

a droplet generator configured to generate droplets of a fuel liquid;

a laser configured to produce a laser beam, the laser arranged to directthe laser beam to hit the droplets generated by the droplet generator atan ignition location;

a source chamber in which the ignition location is located, the sourcechamber having an inner wall with an aperture to pass the radiation outof the radiation source;

a pumping system configured to pump away gas; and

an outlet connected to the pumping system, the outlet located at or nearthe aperture and located outside the inner wall.

According to an aspect, there is provided a lithographic apparatusarranged to project a pattern from a patterning device onto a substrate,the lithographic apparatus comprising:

an illumination system configured to condition radiation from aradiation source, the illumination system having a wall with an apertureto allow radiation from the radiation source to pass into theillumination system;

a suppression flow system configured to provide a gas flow to helpprevent contamination particles from passing through the aperture in thedirection of the radiation, the suppression flow system comprising aflow distributing structure configured to enhance flow homogeneity in adirection transverse to the flow; and

a support structure constructed to hold the patterning device, thepatterning device being capable of imparting the radiation with apattern in its cross-section to form a patterned radiation beam. Theflow may comprise one or more gases from the group consisting of: H₂,He, Ne, Kr and Ar.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 schematically depicts a side view of an EUV illumination systemand projection optics of a lithographic projection apparatus accordingto an embodiment of FIG. 1;

FIGS. 3 a and 3 b schematically depict a set-up of a laser-producedplasma source;

FIG. 4 schematically depicts an interface between a radiation system andan illuminator of the lithographic apparatus of FIG. 1;

FIG. 5 schematically depicts a radiation system, an illuminator and apumping system of the lithographic apparatus of FIG. 1;

FIG. 6 schematically depicts the interface of FIG. 4;

FIG. 7 shows results of an estimation of relative difference of sourcepressure and the Péclet number when varying predetermined parameters;

FIGS. 8-11 schematically show further embodiments of the interface ofFIG. 4; and

FIG. 12 schematically depicts a modification to the radiation system,the illuminator and the pumping system of FIG. 5.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. EUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask or a reticle) MA and connected to a firstpositioner PM configured to accurately position the patterning device;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and

a projection system (e.g. a reflective projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” may encompass any type of projectionsystem, including refractive, reflective, catadioptric, magnetic,electromagnetic and electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, or forother factors such as the use of an immersion liquid or the use of avacuum. It may be desired to use a vacuum for EUV or electron beamradiation since a gas may absorb too much radiation or electrons. Avacuum environment may therefore be provided to the whole beam path withthe aid of a vacuum wall and one or more vacuum pumps.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device supportstructures). In such “multiple stage” machines, the additional tablesand/or support structures may be used in parallel, or preparatory stepsmay be carried out on one or more tables and/or support structures whileone or more other tables and/or support structures are being used forexposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor IF1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam B.Patterning device (e.g. mask) MA and substrate W may be aligned usingpatterning device alignment marks M1, M2 and substrate alignment marksP1, P2.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g. mask table) MT and thesubstrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (e.g. mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g. mask table) MT may be determinedby the (de-)magnification and image reversal characteristics of theprojection system PS.

3. In another mode, the support structure (e.g. mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above. Combinationsand/or variations on the above described modes of use or entirelydifferent modes of use may also be employed.

FIG. 2 shows the projection apparatus 1 in more detail, including aradiation system 42, an illumination system 44, and the projectionsystem PS. The radiation system 42 includes the radiation source SO,which may be a discharge plasma. EUV radiation may be produced by a gasor vapor, for example Xe gas, Li vapor or Sn vapor in which a very hotplasma is created to emit radiation in the EUV range of theelectromagnetic spectrum. The very hot plasma is created by causing anat least partially ionized plasma by, for example, an electricaldischarge. A partial pressure of, for example, 10 Pa of Xe, Li, Sn vaporor any other suitable gas or vapor may be required for efficientgeneration of the radiation. In an embodiment, a Xe, Li, or Sn source asEUV source is applied. The at least partially ionized plasma may beproduced by hitting, for example, Sn with a suitable: laser beam. Insuch an embodiment, a partial pressure of, for example, 0.75 Pa of Xe,Li, or Sn vapor at a temperature of 1200° C. or any other suitable gasor vapor may be required for efficient generation of the radiation. Theradiation emitted by radiation source SO is passed from a source chamber47 into a collector chamber 48 via an optional contaminant barrier 49which is positioned in or behind an opening in source chamber 47. Thecontaminant barrier 49 may comprise a channel structure. Contaminantbarrier 49 may additionally or alternatively comprise a gas barrier.Contaminant barrier 49 may be a combination of a gas barrier and achannel structure. The contaminant barrier 49 further indicated hereinat least comprises a channel structure, as known in the art.

The collector chamber 48 includes a radiation collector 50 (herein alsoindicated as a collector mirror) which may be a grazing incidencecollector. Radiation collector 50 has an upstream radiation collectorside 50 a and a downstream radiation collector side 50 b. Radiationpassed by collector 50 can be reflected off a grating spectral filter 51to be focused in a focal point, referred to as virtual source point 52at an aperture in the collector chamber 48. From collector chamber 48, abeam of radiation 56 is reflected in illumination system 44 via normalincidence reflective elements 53, 54 onto a reticle or mask positionedon reticle or mask table MT. A patterned beam 57 is formed which isimaged in projection system PS via reflective elements 58, 59 ontosubstrate table WT. More elements than shown may be present inillumination system 44 and/or projection system PS. Grating spectralfilter 51 may optionally be present, depending upon the type oflithographic apparatus. Further, there may be more reflective elementspresent than those shown in the Figures, for example there may be 1-4more reflective elements present in the projection system thanreflective elements 58, 59.

In an embodiment, the collector 50, as described herein more detail, isa nested collector with reflectors 142, 143, and 146, as schematicallydepicted in FIG. 2, for example, and is used herein as an example of acollector (or collector mirror). Where applicable, collector 50 as agrazing incidence collector may also be considered as a collector ingeneral and, in an embodiment, as a normal incidence collector.

Instead of a grazing incidence mirror as collector 50, a normalincidence collector 50′ may be applied as is shown in FIG. 3 a. In FIG.3 a, the plasma is produced by hitting Sn, supplied using a dropletgenerator 51, with a suitable laser beam LA, for instance a CO₂ laserLA. Such an arrangement is commonly known as a laser-produced plasma(LPP) source SO. Desirably, a normal incidence collector 140 is used incombination with a LPP source SO.

FIG. 3 b shows the same general arrangement as does FIG. 3 a. However,in FIG. 3 b the contaminant barrier 49 is shown which is similar to thecontaminant barrier 49 disclosed with reference to FIG. 2.

Additionally or alternative to a grating spectral filter 51, asschematically depicted in FIG. 2, a transmissive optical filter may beapplied, or in an embodiment, no filter may be used at all. An opticalfilter transmissive for EUV and less transmissive for or evensubstantially absorbing of UV radiation is known in the art. “Gratingspectral filter” is herein further indicated as “spectral filter” whichincludes a grating and/or transmissive filter. Not depicted in schematicFIG. 2, but included as an optional optical element may be an EUVtransmissive optical filter, for instance arranged upstream of collector50, or an optical EUV transmissive filter in illumination system 44and/or projection system PS.

FIG. 4 shows an interface 60 between portions of the source SO andilluminator IL of an embodiment of the lithographic apparatus 1. Theinterface 60 comprises an inner wall 62 of the radiation system 42 andan inner wall 64 of the illuminator IL. As will be appreciated the innerwall 62 of the radiation system 42 may be the outer wall of theilluminator IL or the inner wall 64 of the illuminator IL may be theouter wall of the radiation system 42. A part of the wall 62 in avicinity of the virtual source point 52, may taper in a direction towardthe virtual source point 52 as is the case in the embodiment shown inFIG. 4. The part of the wall 62 may be cone-shaped. Also, a part of thewall 64 of the illuminator IL in the vicinity of the virtual sourcepoint 52 may taper in a direction toward the virtual source point 52(see FIG. 4). The part of this wall 64 may also be cone-shaped. In thisembodiment both the part of the wall 62 and the part of the wall 64 arewithin a radius of 0.15 m of the virtual source point 52. The part ofthe wall 64 is within a radius of 0.10 m of the virtual source point 52.In FIG. 4 the cone-shaped parts are connected with each other by atubular part 65.

When the lithographic apparatus 1 is in operation, pressure in theradiation system 42 may typically be about 40 Pa, while pressure in theilluminator IL may be about 3 Pa. Due to the pressure difference betweenthe radiation system 42 and the illuminator IL, contamination particles,particularly HBr, may enter the illuminator IL and contaminate theillumination optics, such as normal incidence reflectors 53, 54.

To suppress the contamination, a suppression flow system may beprovided, such as a gas supply 66 at the interface 60. The gas supply 66is configured to provide a gas flow 68, desirably to a location near thevirtual source point 52, to help prevent contamination particles frompassing through the interface 60 in the direction of the radiation beam.The gas flow may comprise one or more gases selected from the groupconsisting of H₂, He, Ne, Ar and Kr. Alternatively or additionally, oneor more other suitable gases may be used. In an embodiment, the gas flowsubstantially consists of H₂, since H₂ absorbs EUV radiation to a lesserextent than He, Ne, Ar and Kr.

To help maintain the radiation system 42 and the illuminator IL at asuitable pressure, a pumping system 70 is provided as shown in FIG. 5.In FIG. 5, the pump system is shown to include four pumps. However, lessor more pumps may be provided. In this embodiment, two pumps 72, 74 pumpaway gas at the radiation system 42, one pump 76 pumps away gas at theilluminator and one pump 78 pumps away gas located at or near theinterface 60 (e.g., virtual source point 52). In this example, pump 78pumps away gas through separate tubular structure 79. As seen in FIG. 4,the outlet of the tubular structure 79 is located between the wall 64and the wall 62. So, for example, the outlet may be located between theinner wall 64 of the illumination system facing away from the radiationsource and an outer wall 62 of the illumination system facing toward theradiation source, or between the inner wall 62 of the radiation sourceand an outer wall 64 of the radiation source facing away from theradiation source, or, where there is both the radiation source and theillumination system, between the inner wall 64 of the illuminationsystem and an inner wall 62 of the radiation source. Thus, the gas ispumped away from the illuminator and the radiation system 42, whichincludes the source SO. Pump capacity at the interface relaxes thepressure specification of the radiation system. For instance, in aconfiguration, adding one small turbo pump at the interface 60 couldsave four large blowers at the source SO.

A contaminant trap (e.g.; a stationary foil trap) 80 may be included inthe radiation system 42 between the radiation source SO and theinterface 60 or virtual source point 52 to help relieve the gas loadfrom the plasma towards the interface 60 or virtual source point 52.Alternatively or additionally, another structure may be used that issubstantially transparent to EUV radiation to similarly help restrictgas passing. Such a structure would, similar to the contaminant trap 82,typically be located between the source SO and the interface 60 orvirtual source point 52.

FIG. 6 is a view similar to FIG. 4. In FIG. 6, however, certaindimensions are indicated, such as angle φ defining the so-called outerNA, the diameter D_(IF) of the tubular part 65, the distanceL_(top source cone IF) between the virtual source point 52 and the topof the cone at the source side of the virtual source point 52 and thedistance L_(base source cone IF) between the virtual source point 52 andthe base of the cone at the source side of the virtual source point.

Table 1 shows exemplary dimensions and other parameters. In Table 1,Q_(count) is the suppression counterflow, also indicated in FIG. 6.S_(IF) is the pumping speed of the pump 78, P_(IF) is the pressure at ornear the virtual source point 52, P_(IL) is the pressure in theilluminator IL, T is the temperature, L_(top illuminator cone IF) is thedistance between the virtual source point 52 and the top of the cone atthe illuminator side of the virtual source point 52,L_(base illuminator cone IF) is the distance between the virtual sourcepoint 52 and the base of the cone at the illuminator side of the virtualsource point, C_(illuminator cone), C_(IF), C_(source cone) are theconductivities of the cone-shaped part of the illuminator-side wall, thetubular part 65, and the cone-shaped part of the radiation system-sidewall respectively, A_(source cone) is an effective surface cone-shapedpart of the radiation system-side wall, and outer NA equals η·sin≈sin φ,because the index of refraction is about equal to unity. Desirably,L_(top illuminator cone IF)+L_(top source cone IF)≈D orL_(top illuminator cone IF)+L_(top source cone IF)<D.

TABLE 1 Q_(count) [Pa · m³/s] 2 S_(IF) [m³/s] 0.5 Outer NA 0.16 P_(IF)[Pa] 1.5 P_(IL) [Pa] 3 T [K] 300 D_(IF) [m] 0.004 C_(illuminator cone)[m³/s] 0.064 C_(IF) [m³/s] 0.01 C_(source cone) [m³/s] 0.127L_(base source cone IF) [m] 0.116 L_(top source cone IF) [m] 0.008L_(base illuminator cone IF) [m] 0.09 L_(top illuminator cone IF) [m]0.03 P · D [Pa · m²/s] 7 A_(source cone)[m²] 0.00023

Also, the following equations can be derived:

$\begin{matrix}{{P_{s} = {\frac{P_{IF}S_{IF}}{C_{IF}} - \frac{C_{illuminator\_ cone}( {P_{source\_ cone} - P_{IF}} )}{C_{IF}} + \frac{S_{IF}P_{IF}}{C_{source\_ cone}} - \frac{Q_{count}}{C_{source\_ cone}} - \frac{C_{illuminator\_ cone}( {P_{source\_ cone} - P_{IF}} )}{C_{source\_ cone}}}}{and}} & ( (2) ) \\{{P\; é} \cong {\frac{L_{sc}}{A_{sc}({PD})}( {Q_{cont} - {P_{if}S_{if}}} )}} & (3)\end{matrix}$

P_(s) is the source pressure and Pé is the so-called Péclet number whichis a measure for the amount of suppression of the mass diffusion of thecontamination particles. Using equations 2 and 3, it can be estimatedhow the source pressure P_(s) and Péclet number Pé will change whenslightly changing one of the input parameters, say by a 5% increase.

The results are shown in FIG. 7. It can be seen that the increase in thepumping speed S_(IF) and the pressure P_(IF) at or near the virtualsource point 52 will increase the source pressure P_(s), and decreasethe Péclet number Pé. To compensate for that, the suppressioncounterflow Q_(count) should be increased. The decrease of the diameterD_(IF) of the tubular part 65 will increase the source pressure P_(s)and suppression.

In order to help prevent the formation of an instability in the gas flowin the cone-shaped parts of the inner walls 62, 64, the aforementionedsuppression flow system may comprise, possibly in addition to the supply66, a flow distributing structure arranged and configured to enhanceflow homogeneity in a direction transverse to the flow. Such a flowdistributing structure may comprise one or more porous media, a sieveand/or any other suitable component or structure.

FIG. 8 shows a possible flow distributing structure 84. The gas issupplied to the flow distributing structure 84 by the supply 66. In thisexample, the flow distributing structure comprises a set of nozzles 86arranged to improve homogeneity and smoothness in the direction Ttransverse to the flow. Consequently, laminarity of the flow may beimproved. To avoid appearance of an instability in gas when exiting theflow distributing structure 84, the gas that is supplied to thestructure 84 has a flow velocity which is lower than the velocity ofsound in that gas. The cone-shaped part of the wall 62 may beconstructed to have a low flow resistance to allow the pressure of thegas when entering this cone-shaped part to be about the same as thepressure of the gas when exiting this cone-shaped part.

In the example of FIG. 10, a supersonic flow may be used in a diffuserby supplying the gas at sonic or supersonic velocity. This may allow fora high velocity in the cone-shaped part of the wall 62 to cause a largeamount of suppression. Also, the large flow velocity may cause a shockwave which is difficult for possible contaminants to pass.

FIG. 9 shows an embodiment of a possible flow distributing structure 84.The cone-shaped part of wall 62 does not taper towards the virtualsource point 52, but rather to the source SO. In this example, the flowdistributing structure 84 is provided with a set of nozzles 86 to directflow towards the source. The set of nozzles 86 extends in a directiontransverse to the flow direction. As in the example of FIG. 8 in whichthe structure 84 was a diffuser, here the set of nozzles of the flowdistributing structure 84 helps improve homogeneity and smoothness inthe direction T transverse to the flow. Thus, laminarity of the flow maybe improved. Desirably, the flow velocity in the diffuser is smallerthan the velocity of sound in the gas.

In the embodiment of FIG. 10, it can be seen that, when viewing from thevirtual source point 52 to the source SO, the cross-section of the wall62 is increased step-by-step. At each step, a flow of gas is supplied tocompensate for the flow speed decrease on one hand and to help minimizethe presence of an instability on the other. Such a configuration mayprovide a constant, laminar and high velocity flow towards the sourceSO. Furthermore, additional suppression of contaminant diffusion alongthe wall 62 may be prevented.

In a further embodiment, a so-called purge hood construction as shown inFIG. 11 is provided. This would allow for flat walls 62. A porousmaterial can be used at the walls 62, the material forming the flowdistributing structure 84. Such a configuration allows for combining asmall flow velocity with a high mass flow rate through the flowdistributing structure 84. The small flow velocity allows for the flowto become laminar quickly after leaving the flow distributing structure84 and the high mass flow rate allows for a high amount suppression,i.e. high Péclet number Pé.

Generally the presence of the flow distributing structure 84 asdiscussed above allows for a smaller size of the interface between theradiation system 42 and the illuminator IL. The suppression flow systemmay comprise the flow distributing structure arranged and configured toenhance flow homogeneity in a direction transverse to the flow withoutthe presence of a pumping system configured to pump away gas at or nearan aperture to allow radiation from the radiation source into theillumination system.

A modification to the embodiments of the lithographic apparatus is shownin FIG. 12. In FIG. 12, all but one of the aforementioned pumps 72, 74,76 and 78 is shown. The lithographic apparatus shown in FIG. 12comprises a thermal control system configured to maintain a temperaturedifference between at least two locations in the radiation system.Maintaining such a temperature difference may allow for a higherpressure in the radiation system 42. In the example of FIG. 12, thecontrol system comprises a controller C, a first temperature sensor 88,a second temperature sensor 90 and a heat exchanger 92. In this example,the controller C is configured to activate the heat exchanger 92 basedon the temperature measured by the first temperature sensor 88 and thesecond temperature sensor 90. Desirably, the temperature measured by thefirst temperature sensor 88 is maintained low relative to thetemperature measured by the second temperature sensor 90. This willallow for a relatively low temperature between the source SO and thecollector 50′ mirror to stop fast ions, while the temperature in theremaining part of the radiation system 42 is allowed to remain high toallow for good transmission.

The principle can be understood using the following equation:

$\begin{matrix}{{Tr} = {\exp ( {- \frac{p \cdot \sigma \cdot L}{k \cdot T}} )}} & (4)\end{matrix}$

Tr is transmissivity of the gas to extreme ultraviolet radiation, P isthe partial pressure of a certain absorbing gas, σ is the absorbingcross-section of the gas, L is the length of the radiation path forwhich the transmissivity is to be determined, k is the Boltzmanconstant, and T is the temperature. From equation 4, it can be seen thatthe transmissivity depends on the ratio P/T rather than P by itself.Therefore, by maintaining a certain temperature difference betweencertain locations in the radiation system 42, the total transmissivityalong an optical path of the radiation emitted by the source SO can bemaintained at a suitable level while allowing for relatively highpressures at locations where such high pressures may be desirable.

Thus, when the temperature is maintained at a low level between thesource SO and the collector 50′, the high pressure will help stop thefast ions, while the loss in transmissivity is compensated in thevicinity of the second temperature sensor 90 where the high pressure iscompensated using a high temperature.

Such a control system may be used in the absence of a pumping systemconfigured to pump away gas at or near an aperture to allow radiationfrom the radiation source into the illumination system. In addition toor instead of the heat exchanger 92, a heating structure may beprovided, for instance at a position near the second temperature sensor90.

In an embodiment, there is provided a lithographic apparatus arranged toproject a pattern from a patterning device onto a substrate, thelithographic apparatus comprising: an illumination system configured tocondition radiation from a radiation source, the illumination systemhaving an inner wall facing away from the radiation source and an outerwall facing toward the radiation source, the inner and outer wallshaving an aperture to allow radiation from the radiation source to passinto the illumination system; a pumping system configured to pump awaygas; an outlet connected to the pumping system, the outlet located at ornear the aperture and located between the inner wall and the outer wallof the illumination system or, if the lithographic apparatus comprises aradiation source, between the inner wall of the illumination system andan inner wall of the radiation source facing toward the radiation sourceand adjacent the inner wall of the illumination system; and a supportstructure constructed to hold the patterning device, the patterningdevice being capable of imparting the radiation with a pattern in itscross-section to form a patterned radiation beam.

In an embodiment, the outlet is located at or near a focal point of theradiation. In an embodiment, the lithographic apparatus furthercomprises the radiation source and a collector configured to focusradiation emitted by the radiation source into a focal point. In anembodiment, a part of the outer wall of the illumination system or, ifthe lithographic apparatus comprises a radiation source, the inner wallof the radiation source is tapered in a direction toward the focalpoint. In an embodiment, the part is cone shaped. In an embodiment, thetapered part is within a radius of 0.15 m of the focal point. In anembodiment, the outlet of the pumping system extends through the part.In an embodiment, the lithographic apparatus further comprises asuppression flow system configured to provide a gas flow to help preventcontamination particles from passing through the aperture in thedirection of the radiation. In an embodiment, the gas flow comprises oneor more gases selected from the group consisting of: H₂, He, Ne, Kr andAr. In an embodiment, the suppression flow system comprises a flowdistributing structure arranged and configured to enhance flowhomogeneity in a direction transverse to the flow. In an embodiment, thelithographic apparatus further comprises: a substrate table constructedto hold the substrate; and a projection system configured to project thepatterned radiation beam onto a target portion of the substrate. In anembodiment, a part of the inner wall of the illumination system istapered in a direction toward a focal point of the radiation. In anembodiment, the part of the inner wall of the illumination system iscone shaped. In an embodiment, the tapered part of the inner wall iswithin a radius of 0.15 m of the focal point. In an embodiment, thetapered part of the inner wall is within a radius of 0.10 m of the focalpoint. In an embodiment, the outlet of the pumping system extendsthrough the part of the inner wall of the illumination system.

In an embodiment, there is provided a lithographic apparatus arranged toproject a pattern from a patterning device onto a substrate, thelithographic apparatus comprising: an illumination system configured tocondition radiation from a radiation source, the illumination systemhaving a wall with an aperture to allow radiation from the radiationsource to pass into the illumination system; a suppression flow systemconfigured to provide a gas flow to help prevent contamination particlesfrom passing through the aperture in the direction of the radiation, thesuppression flow system comprising a flow distributing structureconfigured to enhance flow homogeneity in a direction transverse to theflow; and a support structure constructed to hold the patterning device,the patterning device being capable of imparting the radiation with apattern in its cross-section to form a patterned radiation beam.

In an embodiment, the gas flow comprises one or more gases from thegroup consisting of: H₂, He, Ne, Kr and Ar.

In an embodiment, there is provided a radiation source arranged to emitradiation, the source comprising: a droplet generator configured togenerate droplets of a fuel liquid; a laser configured to produce alaser beam, the laser arranged to direct the laser beam to hit thedroplets generated by the droplet generator at an ignition location; asource chamber in which the ignition location is located, the sourcechamber having an inner wall with an aperture to pass the radiation outof the radiation source; a pumping system configured to pump away gas;and an outlet connected to the pumping system, the outlet located at ornear the aperture and located outside the inner wall.

In an embodiment, the outlet is located at or near a focal point of theradiation. In an embodiment, a part of the inner wall is tapered in adirection toward a focal point of the radiation. In an embodiment, thepart of the inner wall is cone shaped. In an embodiment, the sourcechamber comprises an outer wall opposite the inner wall and wherein apart of the outer wall is tapered in a direction toward a focal point ofthe radiation. In an embodiment, the radiation source further comprisesa suppression flow system configured to provide a gas flow to helpprevent contamination particles from passing through the aperture in thedirection of the radiation, wherein the gas flow comprises one or moregases selected from the group consisting of: H₂, He, Ne, Kr and Ar. Inan embodiment, the suppression flow system comprises a flow distributingstructure arranged and configured to enhance flow homogeneity in adirection transverse to the flow.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1.-20. (canceled)
 21. A radiation source system, comprising: a dropletgenerator configured to generate droplets of a fuel liquid; a laserconfigured to produce a laser beam, the laser arranged to direct thelaser beam to hit the droplets generated by the droplet generator at anignition location; a source chamber in which the ignition location islocated, the source chamber having an inner wall with an aperture topass the radiation out of the source chamber; a pumping systemconfigured to pump away gas; and an outlet connected to the pumpingsystem and the source chamber, the outlet located at or near theaperture and located outside the inner wall.
 22. The radiation system ofclaim 21, wherein the outlet is located at or near a focal point of theradiation.
 23. The radiation system of claim 21, wherein a part of theinner wall is tapered in a direction toward a focal point of theradiation.
 24. The radiation system of claim 23, wherein the part of theinner wall is cone shaped.
 25. The radiation system of claim 23, whereinthe source chamber comprises an outer wall opposite the inner wall andwherein a part of the outer wall is tapered in a direction toward afocal point of the radiation.
 26. The radiation system of claim 21,further comprising a suppression flow system configured to provide a gasflow to help prevent contamination particles from passing through theaperture in the direction of the radiation, wherein the gas flowcomprises one or more gases selected from the group consisting of: H₂,He, Ne, Kr and Ar.
 27. The radiation system of claim 26, wherein thesuppression flow system comprises a flow distributing structure arrangedand configured to enhance flow homogeneity in a direction transverse tothe flow.
 28. A method, comprising: producing radiation in a sourcechamber having an inner wall with an aperture to pass the radiation outof the source chamber, the producing radiation comprising generatingdroplets of a fuel liquid and directing a laser beam to hit thegenerated droplets at an ignition location in the source chamber; andpumping away gas through an outlet that is connected to the chamber,located at or near the aperture, and located outside the inner wall. 29.The method of claim 28, wherein the outlet is located at or near a focalpoint of the radiation.
 30. The method of claim 28, wherein a part ofthe inner wall is tapered in a direction toward a focal point of theradiation.
 31. The method of claim 28, further comprising providing agas flow to help prevent contamination particles from passing throughthe aperture in the direction of the radiation, wherein the gas flowcomprises one or more gases selected from the group consisting of: H₂,He, Ne, Kr and Ar.
 32. The method of claim 31, comprising providing thegas flow using a flow distributing structure configured to enhance flowhomogeneity in a direction transverse to the direction of the radiation.33. A device manufacturing method, comprising: conditioning radiation,from a radiation source, using an illumination system having a wall withan aperture to allow radiation from the radiation source to pass intothe illumination system; providing a gas flow, at or near the aperture,to help prevent contamination particles from passing through theaperture in the direction of the radiation, wherein the gas flow isprovided by a plurality of gas flow inlets extending in a directiontransverse to the direction of the radiation to enhance flow homogeneityin the direction transverse to the direction of the radiation; andprojecting the radiation onto a radiation-sensitive substrate.
 34. Themethod of claim 33, wherein the illumination system has an inner wallfacing away from the radiation source and an outer wall facing towardthe radiation source, the inner and outer walls having the aperture. 35.The method of claim 33, further comprising pumping away gas through anoutlet located at or near the aperture and located between an inner walland an outer wall of the illumination system or between an inner wall ofthe illumination system and an inner wall of the radiation source facingtoward the radiation source and adjacent the inner wall of theillumination system.
 36. The method of claim 33, wherein the gas flowcomprises one or more gases from the group consisting of: H₂, He, Ne, Krand Ar.
 37. The method of claim 33, wherein a part of the wall istapered in a direction toward a focal point of the radiation and thetapered part is within a radius of 0.15 m of the focal point.
 38. Adevice manufacturing method, comprising: conditioning radiation, from aradiation source, using an illumination system having a wall with anaperture to allow radiation from the radiation source to pass into theillumination system, the aperture having a tapered part; providing a gasflow to help prevent contamination particles from passing through theaperture in the direction of the radiation, wherein the gas flow isprovided by a flow distributing structure, extending along the taperedpart, having a two-dimensional array of inlets in or on the tapered partto supply gas to the aperture; and projecting the radiation onto aradiation-sensitive substrate.
 39. The method of claim 38, wherein theflow distributing structure comprises a flat wall of porous media. 40.The method of claim 38, further comprising pumping away gas through anoutlet located at or near the aperture.